This invention relates generally to tufting machines for forming fine gauge cut pile fabric and more particularly to improved knife blocks for mounting the knives in such machines, especially in the staggered needle variety of such machines.
In cut pile tufting machines an oscillating knife cooperates with an oscillating looper to cut the loop of yarn that has been seized by the looper from a corresponding needle. Until recently the prior art knife blocks were limited to mounting one or two knives only. However, the current trend in cut pile carpeting has been increasingly directed toward fine guage fabric, this being one tenth gauge and smaller. Since the gauge of a pile fabric is determined by the spacing between adjacent gauge parts, i.e., the longitudinally adjacent needles, loopers and knives, for fine gauge fabrics, the spacing between a point on one knife to the corresponding point on an adjacent knife is 0.1 inch and smaller. Thus, the thickness of the web of a knife block carrying two knives in a 1/10 gauge machine must be 0.1 inch minus the thickness of one knife and the maximum overall thickness of the block would be 0.2 inch, which then is the maximum diameter of the shaft supporting the block in the knife bar. This is considered to be too small to support the load on the block. Therefore, knife blocks carrying more than two blades have been proposed which allows use of a larger diameter support shafts.
Fine gauge cut pile tufting machines can be manufactured having all the needles in a single longitudinal row or the needles may be placed in parallel rows laterally spaced and staggered one from the other. The later approach is advantageous when the thicker yarns are to be tufted. A machine of this type is illustrated in Crumbliss et al U.S. Pat. No. 3,913,505. In the machine illustrated in that patent the throats of the loopers and therefore the knives which cut at the throats are all respectively aligned. In other machines of the staggered needle type the throats of alternate loopers are staggered to the same extent as the needles thus, since the knives cut at the respective throats of the loopers, adjacent knives are offset by the amount of the stagger. See, for example Card U.S. Pat. No. 3,084,644.
A manufacturing problem arises when a knife block carrying more than two knives and particularly four knives, is utilized with a staggered needle arrangement having loopers with alternate throats offset and where the front to rear stagger is greater than a minimal amount. With a conventional four bladed block having aligned knives, each knife receiving channel is a slot cut with a milling cutter. Since the width of the knives are standard at approximately 1/2 inch, the mill cutter diameter is approximately 1/2 inch and the shank diameter of the cutter tool is approximately 3/8 inch. However, when the front to back needle stagger is approximately 1/4 inch, the inner slots of a four knife block, i.e., the slots adjacent the web, cannot be milled because the maximum depth of staggered cut allowed by the shank acting against the metal about the outer slots is approximately 1/16 inch, the difference between the cutter radius and the shank radius. In fact, with a 1/4 inch knife stagger, a one half inch cutter cannot be used at all because there would be no space for a shank of any diameter. If a knife of a larger width were to be used the knife would be too stiff for use with the fine gauge looper.